* YAM_UT.c, WriteWindow.c: implemented a function to check whether a string is correctly UTF8 encoded and contains at least one UTF8 character. Based on this check the charset of text attachments is forced to either UTF8 or ISO-8859-1 instead of the configured write mail charset. This refs #600. Please note that YAM does NO reencoding, it just gives the receiver a hint how to handle the attached file.